In high water cut and extra high water cut exploitation periods, water flooding in huge amounts is necessary for stabilizing oil yields. Long-term water flooding leads to channeling through large passages, so the injection water cycles with very low efficiency result in increased costs of oil recovery. Consequently, effective water flooding has become one of the most important measures for the exploitation of high water cut and extra high water cut oilfields at low cost.
From laboratory studies and field tests over the years, it has been realized that water flooding itself can result in fairly high displacement efficiency, and that the key to further improving the recovery effect by water flooding is to improve water sweep efficiency. The principle problem of anaphase of the high water cut period is the futile cycle of injection water due to the initial heterogeneity of the oil reservoir and the “aggravating heterogeneity” caused by long-term water flooding. To solve the problem, it is not pragmatic to thoroughly infill the well pattern or to largely improve the water intake per unit thickness. Only by reforming the set streamline field formed by long-term water flooding to transfer the injection water to low-permeability zones with low sweeping efficiency can one control the huge futile cycle of the injection water through high permeability zones with high sweep efficiency.
Owing to the complexity of high water cut anaphase in an oil reservoir, especially when “large passages” have appeared in the oil reservoir, studying the distribution of residual oil has no practical guiding significance on the effective improvement of waterflooding sweep volume. Current technologies of profile control and in-depth profile control can only solve the water-channeling problem in nearby wellbore zones and cannot provide a satisfactory answer for the water channeling problem in in-depth areas of a reservoir. Studies should be concentrated on the flowing direction and velocity of water (namely, the present streamline field of water in the oil reservoir), to acquire the current situation and streamline field of water flooding. Based on the current understanding of water flooding (e.g. streamline distribution) in a reservoir, the streamline field may be reformed by developing new methods and effective technologies for the diversion of injection water in in-depth areas of a reservoir to efficiently increase the waterflooding sweep volume. What is critical is to develop a new in-depth diverting agent, rather than a profile-controlling agent.
Much prior art exists regarding the realization of in-depth flooding fluid diversion. U.S. Pat. No. 4,787,451 and Chinese Patent Application No. 00103532.0 disclose in-depth flooding fluid diversion using an underground cross-linking weak gel system with acrylamide polymer as the main material. Chinese Patent Application No. 00100888.9 discloses in-depth flooding fluid diversion using an underground cross-linking colloidal dispersion gel system with acrylamide polymer as the main material. Chinese Patent Application No. 98120664.6 discloses in-depth flooding fluid diversion by using a pre-cross linking swellable particle system with acrylamide polymer as the main material. In recent years, the in-depth flooding fluid diversion technology with the adoption of aforesaid systems has yielded certain results in raising the water flooding efficiency, and has become a useful measure of the improvement of exploitation reserves and enhanced oil recovery.
However, as is known, acrylamide may be hydrolyzed and produce acrylic acid (acrylate) under certain temperatures or in acid and alkaline conditions. The polymerizing reaction for preparing acrylamide polymer from acrylamide as the main material is exothermic, and will generate a large amount of heat. The reaction heat cannot be released since the generated acrylamide polymer transforms the reaction system promptly into a gel phase, leading to the rapid increase of the reaction system temperature. As a result, the acrylamide polymer product is, in essence, the copolymer of acrylamide and acrylic acid (acrylate). In fresh water, because of the electric mutual repulsion between the carboxyl sodium groups within the acrylamide polymer molecules, the acrylamide polymer molecules are in an extending state and have a strong viscosity-increasing ability. In brine, because the electric property of the carboxy sodium groups within the acrylamide polymer molecules is shielded, the acrylamide polymer molecules are in a curling state. The higher the degree of hydrolysis (i.e., the higher the content of carboxy sodium groups), the more the acrylamide polymer molecules curl in brine and the lower the polymer's viscosity increasing ability will be. Hydrolysis reactions will occur continuously in acrylamide polymer molecules under stratum temperature. When the degree of hydrolysis of acrylamide polymer is ≧40%, no precipitant will appear even for the serious curling of the acrylamide polymer molecules and the great reduction of its viscosifying ability. However, in hard water (i.e., high Ca2+ and/or Mg2+ content), when the hydrolysis degree of acrylamide polymer is ≧40%, flocculant precipitants will appear because the acrylamide polymer molecules combine with multiple valence ions such as Ca2+ and Mg2+.
The four above-mentioned patents all teach employing acrylamide polymer as a major material without teaching long-term stability regarding the properties of the raw material. Additionally, the placement of an in-depth flooding fluid-diverting agent is a technology for allowing the in-depth flooding fluid diverting agent to get into in-depth positions where “large passages” or high permeability zones exist in reservoirs. Analyzed from the aspect of placement technology, when an underground cross-linking placement technology is employed, an optimized cross-linking condition in ground design cannot be accomplished completely, and great risk will result in reaching successful cross-linking underground. This is because the pH, salinity and temperature of the stratum all influence the cross-linking system, the hydrolysis degree varies constantly, and adsorption and retention of the polymer on the surface of stratum also affect the valid frontier concentration of the polymer and the chromatographic separation of the cross-linking system by the stratum. Also, the effective distance of underground cross-linking placement technology is restrained for the reason that effective gelation time cannot be delayed for very long. Furthermore, it is difficult for the underground cross-linking system to meet the needs of in-depth flooding fluid diversion, for it either hardly deforms with high strength or is crushed with low strength. In the ground cross-linking placing technology, the adsorption and wall-holding ability of gel is low after interaction between the absorbing groups, which leads to weaker transporting resistance through “large passages”, and thus the ability of in-depth flooding fluid diversion is compromised. Moreover, the same problems as in the underground cross-linking system also exist in this system. In a word, these systems exhibit drawbacks of low deformation ability, being unable to resist heat or salt and having no long-term stability etc., and these drawbacks affect the implementation and economic benefits of in-depth flooding fluid diversion technology.
Accordingly, there exists a need for a new in-depth flooding fluid diverting agent with strong deformation ability, resistance to temperature and salt, and long-term stability.